Large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO2 in a mountain stream

Abstract

Understanding the process of CO2 degassing during precipitation of calcite from a Ca2+–HCO3− solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO2(g) and HCO3− due to breaking of CO bond, with an equilibrium fractionation of ca. −9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO2 in a supersaturated solution where rapid calcite precipitation drives CO2 degassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO2 via HCO3− dehydration and dehydroxylation. Average carbon isotope fractionation between CO2(g) and HCO3− (εCO2(g)- HCO3-) calculated from our dataset is about −20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between εCO2(g)- HCO3- and precipitation rate was observed, indicating higher precipitation rates cause εCO2(g)- HCO3- farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO2(g) and HCO3− may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of εCO2(g)- HCO3- has special implications for speleothem archives from ventilated caves. As partial pressure of CO2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO2 and DIC and thus perturb the time-series of speleothem’s δ13C records for paleo-environmental reconstruction.